Ferrofluids are magnetically responsive materials and consist of three components: magnetic particles, a surfactant and a liquid carrier. The particles, typically Fe.sub.3 O.sub.4, are of submicron size, generally about 100 .ANG. in diameter. The magnetic particles are coated with a surfactant to prevent particle agglomeration under the attractive Van der Waals and magnetic forces and are dispersed in the liquid carrier. Ferrofluids are true colloids in which the particles are permanently suspended in the liquid carrier and are not separated under gravitational, magnetic and/or acceleration forces. The liquid carrier can be an aqueous, an oil or an organic solvent.
Ferrofluids can be utilized in the separation of mixed nonferrous materials or minerals such as those found in auto scrap, machine shop waste and glacier deposits. The separation process is based on the density of the materials and depends on the fact that the ferrofluid generates a magnetic "levitation" force when placed in an inhomogeneous magnetic field. An upward-directed levitation force floats normally sinking particles by counterbalancing their density mismatch with the ferrofluid.
Two different techniques are commonly used to perform the separation process. The first conventional technique is called the magnetostatic or the sink/float process. In this process material to be separated is passed through a static column of ferrofluid situated in a gradient magnetic field. Material of higher density sinks to the bottom and material of lower density floats to the top. When the magnetic field gradient is appropriately adjusted, two fractions are generated which are collected in separate bins.
The second conventional technique is called the magnetodynamic process. In this process a vertical column of ferrofluid is also located in a magnetic field gradient but the fluid is rotating rather than being static. The magnetic field gradient is aligned so that the magnetic levitation force is toward the axis of rotation of the ferrofluid column. A stream of particles to be separated is introduced at the top of the ferrofluid column. As the particles fall under the influence of gravity they are subjected to opposing centrifugal and ferrofluid levitation forces causing the particle stream to split up into two fractions, one of higher density and one of lower density. At the bottom of the column the higher density component is collected farther form the axis of rotation and the lower density component is collected near the rotation axis. Both the sink/float technique and the magnetodynamic technique are described in detail in an article entitled "Separation of Nonmagnetic Particles With Magnetic Fluid", T. Fujita printed in the book Magnetic Fluids and Applications Handbook; ed. B. Berkovski; Begell House, Inc., New York (1996), which article is incorporated in its entirety by reference herein. The sink/float technique is also disclosed in U.S. Pat. No. 3,483,969 which is also incorporated by reference.
Ferrofluids used in material separation processes use a relatively low viscosity carrier liquid such as water, kerosene or a low molecular weight refined hydrocarbon solvent such as Isopar solvent produced by Exxon Corporation, Houston, Tex. The low viscosity of the carrier liquid is necessary for efficient separation. The saturation magnetization of ferrofluid depends on the process and the density of materials to be separated and may range from 10 to 600 Gauss.
In both of the conventional separation techniques the separated material is often coated with ferrofluid and must be washed with a solvent to complete the final step in the process. The waste liquid which results from the washing step may be viewed as a ferrofluid diluted with solvent and contaminated with dust particles and other impurities. Moreover, this dilute ferrofluid is well below the concentration which can be used in the separation process and is, therefore, essentially lost.
Since up to 10 per cent of the ferrofluid used in the separation process may be lost in the washing step, ferrofluids currently are not widely used in nonferrous material separation applications due to high cost of the fluids. However, if both the solvent and the ferrofluid could be reclaimed, the cost of separation process could be considerably reduced.
U.S. Pat. No. 4,435,302 discloses a chemical method for reclaiming and concentration of water-based magnetic fluids. In this patent the separated materials which are coated with ferrofluid are washed in water. The magnetic particles in the dilute washing liquid are chemically flocculated by addition of hydrochloric acid. The flocculant is removed from the liquid by filtration and then redispersed in water to a desired concentration. A problem with this process is that dust and other impurities present in the washing liquid are also separated with the flocculant and remain in the reconstituted ferrofluid, thereby contaminating it. Furthermore, an additional chemical is required for the flocculation step thereby adding to the cost of the process.
Japanese Patent Application No. 52-30973 shows a process for reclamation of an organic liquid based ferrofluid. The coated particles resulting from the separation process are washed with 1,1,1 trichloroethane cleaning solvent which is different from the organic carrier in which the ferrofluid particles are suspended. Both the solvent and ferrofluid can be recovered from the resulting wash liquid by distillation which removes dust and other contaminants. This system is effective but has drawbacks: Vapors from the 1,1,1 trichloroethane cleaning solvent pose a serious health hazard. In addition, even after distillation, traces of the cleaning solvent may be present in the reclaimed ferrofluid and thus may affect its properties. Finally, with such a process, the magnetization of the reclaimed ferrofluid cannot be increased beyond its original value to achieve separation of a wide range of materials.
Accordingly, there is a need for a better ferrofluid reclamation process.